Scale inhibitors are used in production wells to stop scaling in the rock formation and/or in the production lines down hole and at the surface. Scale is a slightly soluble inorganic salt, such as barium or strontium sulphate, calcium carbonate, calcium sulphate or calcium fluoride. In the production of hydrocarbons from subterranean formations the deposition of scale on surfaces and production equipment is a major production problem. Scale build-up decreases permeability of the formation, reduces well productivity and shortens the lifetime of production equipment. In order to clean scaled-up wells and equipment it is necessary to stop the production i.e. by shutting in the well which is time-consuming and costly.
To minimise scale build-up a solution of a scale inhibitor may be injected by force into the formation via a production well-bore. After injection the production well is shut-in during which time the scale inhibitor is absorbed within the formation. After the shut-in period the production well is returned on stream and the inhibitor is slowly desorbed into the fluids in the formation to inhibit scale deposition. The fluids produced therefrom are analysed to determine the scale inhibitor concentration. When the concentration of inhibitor in the fluids has reduced to a certain level then further treatments will be required. An aqueous-based scale inhibitor may have a short lifetime of a few weeks. The continual need for such treatments is therefore costly, not only in terms of production shut down periods but also in the cost of the chemical scale inhibitor used.
Other water-soluble or water-dispersible inhibitors used in production well environments include corrosion inhibitors, hydrogen sulphide scavengers or hydrate inhibitors. These too may need shut-ins.
According to U.S. Pat. No. 5,547,025, it is well-known to those skilled in the art that gelled or crosslinked water-soluble polymers are useful in enhanced oil recovery and other oil field operations. In particular, they have been used to alter the permeability of underground formations in order to enhance the effectiveness of water flooding operations. Generally, polymers along with an appropriate crosslinking system are injected in an aqueous solution into the formation. The polymers then permeate into and gel in the regions of the formation having the highest water permeability. Specifically, the process of U.S. Pat. No. 5,547,025 comprises injecting into a formation a gelling composition which comprises a carboxylate-containing polymer, a crosslinking agent and a liquid wherein the gelling composition forms a gel when injected into the formation.